Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 918

medium and cells upward. At the top, bubbles break out of the medium, dropping cells and
liquid medium. Cells and liquid medium flows down from the annulus on the side and then
being carried upward from the center region (draught tube). Loop reactors can handle more
viscous fluids than bubble columns and applied to situation where bubble coalescence is
generally not a problem.

18 .2. REACTOR OPERATIONAL MODE SELECTIO N

There are three general operation modes for a stirred-tank reactor: batch, continuous
(or CSTR or chemostat), and fed-batch. The features of these (ideal) reactors have been dis-
cussed in Chapters 3, 4, 5, 11, 12, and 13. A summary of the reactor performance features is
given in Table 18.3 . A batch reactor has nearly an identical flow contact pattern as a PFR. There

TABLE 18.3 A Summary of Reactor Operational Modes for a Stirred-Tank or Well-Mixed Reactor

Operational
model Batch CSTR Fed-batch

Substrate Highest concentration in the

reactor, concentration
decreases with residence time
inside the reactor,

Lowest concentration in the
reactor; constant and
controllable concentration in
the reactor

Start-up period similar to
batch, and the bulk of the
operation could mimic
a CSTR

Desired product Lowest concentration in the

reactor, concentration
increases with residence time
in the reactor

Highest concentration in the
reactor. Ability to promote the
desired product selectivity by
controlling the substrate
concentration

Above

By-product Reduced opportunity for

degradation product(s) from
desired product(s)

Reduced opportunity for side-
product formation with
a controlled substrate
concentration

Above

Catalyst and/or
cells

Residence time of catalyst is
controllable and uniform;
catalysts/cells are selected;
complete catalysts replaceable
at each batch

Residence time of suspended
catalysts/cells is not uniform
in the reactor; selection favors
fast growth cells; fractional
catalysts/cells replacement
can be implemented

Same as batch operation

Flexibility Distinctive batches, quality

ensured at each batch level,
easy to change operations

High throughput, easy to
control, consistent quality,
difficult to change operations

Distinctive batches,
moderate throughput,
quality controllable at each
batch, easy to change
operations

Applications Small quantity production,

high value substrate/product,
multiple products/
substrates/processes on
demand; catalyst instability
(mutation)

Large quantity production;
high demand; consistent
substrate supply and product
quality demand; stable
catalyst or controllable
through feed

Process requiring strict
product quality control
while flexible operation is
required or catalyst
unstable with long
duration of operations